I was going to suggest pulling a light bulb from a taillight or under dash...whatever is easy. Hook it up to 12V and if it glows real bright and burns out, it was 6V. The headlight check as well as a red or black tag on the generator are also good checks. The starter will not care 6V or 12V. The solenoid will hit too hard if it is 6V run on 12. The gas gauge, heater motor, headlights, (and other light bulbs) and radio if equipped will object to over voltage.
